Microwave bandpass filters are commonly realized using one or more resonators. Broadly speaking, a resonator is any physical element that stores both magnetic and electric energy in a frequency-dependent way. The resonant frequency of a resonator is defined as any frequency at which the stored electric and magnetic energies in the resonator are equal, and at that frequency the resonator is said to be in resonance.
Realizations of microwave resonators, however, are not so limited. At microwave frequencies, potentially any three-dimensional structure can be used to realize a resonator, in which internal electric and magnetic field distributions are generally determined by the shape and size of the overall structure. Some classes of microwave resonators include lumped element, microstrip, coaxial, waveguide, and dielectric resonators. Each class has application specific advantages and disadvantages.
In general, a dielectric resonator (DR) cavity comprises a dielectric resonator formed in a high-permittivity substrate mounted inside a metallic housing using a mounting support formed in a low-permittivity substrate. Compared to lumped element and microstrip resonators, dielectric resonators (as well as coaxial and waveguide resonators) tend to be bulkier in size and more complex in design, but offer superior Q values. In present microwave technologies, dielectric resonators offer Q values in the range of 3,000 to 40,000 at 1 GHz. For this reason, dielectric resonator filters are often favoured for use in satellite/space communication and wireless base station applications, where low loss and high power can be overriding design considerations. In addition to the Q values, resonator size and spurious performance (the frequency separation between an operating mode of the resonator and adjacent resonant modes) can also be important design considerations
Dielectric resonators are also commonly operated as single-mode resonators, and dual-mode resonators, and less commonly as triple-mode and quadruple-mode resonators. A single-mode resonator supports only a single field distribution at the resonator's center frequency. Correspondingly, a dual-mode resonator supports two different field distributions and a triple-mode resonator supports three different field distributions. The intention for using a higher number of modes is mainly size reduction, as one physical resonator is overloaded with more than one electrical resonator, and each electrical resonator is supported by a mode distribution. Resonance modes, such as dual and triple-modes, which support a plurality of field distributions at the center frequency, are referred to as degenerate modes. In the usual case, the different field distributions in a degenerate mode are orthogonal modes of a similar field distribution and are created due to symmetries in the resonator. Thus, dual modes have been mainly realized with resonators having 90-degree radial symmetry (cylindrical and rectangular waveguide cavities and resonators), while triple modes are supported for example in cubic waveguide cavities and cubic dielectric resonators.
Quadruple-mode dielectric resonators have also been realized, but mainly due to complications in fabrication and tuning, comparatively less interest has been generated in this area. In order to realize a quadruple-mode dielectric resonator, independent or near independent control over the coupling and tuning of each of the four modes is required, which generally results in a complex overall coupling scheme involving a large number of tuning and/or coupling screws. Although tuning and coupling schemes necessary for single-mode and dual-mode dielectric resonators add some design complexity as well, the added design complexities are more pronounced in triple-mode dielectric resonators, and even more pronounced in presently known realizations of quadruple-mode dielectric resonators. Dual-mode, triple-mode, and quadruple-mode resonators remain attractive alternatives to single-mode dielectric resonators, however, because of their associated size reduction, especially considering that dielectric resonators already tend to be bulky. For the applications in which dielectric resonator filters are preferred, e.g. satellite/space systems, size and mass reduction are highly desirable.